Interface DistributedManager

Interface implemented by session managers that do not keep a complete copy
of all sessions in memory but do know where every session is. The
BackupManager is an example of such a Manager as are implementations of the
StoreManager interface.

With the BackupManager, sessions can be primary (master copy on this node),
backup (backup copy on this node) or proxy (only the session ID on this
node). The identity of the primary and backup nodes are known for all
sessions, including proxy sessions.

With StoreManager implementations, sessions can be primary (session is in
memory) or proxy (session is in the Store).